This invention relates to pipettes and titrators and, more particularly, to pipettes having an electrically operated linear actuator. Specifically, the invention is directed to a self-contained automated pipette for portable operation having an electronically controlled digital linear actuator, which accommodates removably attachable pipetting displacement assemblies of various sizes for providing improved precision and accuracy.
Mechanically operated pipettes are known. These pipettes have spring activated stops for controlling displacement piston movement.
Mechanically operated pipettes rely on repeated operator precision because they employ different spring constants for providing tactile sensing of proper displacement piston stroke. Unfortunately, such soft stops are not precise and are often missed due to operator inexperience, fatigue, or inattention. Imprecision in pipetting and/or titrating results. Advantageously, however, mechanically operated pipettes are self-contained, that is, stand alone instruments, and are generally portable.
Electrically operated linear actuators for controlling displacement piston movement in a pipette are known. See Nishi U.S. Pat. No. 3,915,651 which discloses an electronic preset indexer connected to a stepping motor through a cord attachment. The stepping motor is energized by an essentially infinite source of power for driving a screw slide assembly actuated displacement rod.
In order to effectively use a pipette having an electrically operated linear actuator in a laboratory, a portable instrument approaching the size, shape, and weight of known mechanically operated pipettes is desirable. The size and shape of the pipette is critical to portability. If the pipette is overly long, the instrument is unwieldy. Heretofore, electrically operated pipettes have been configured so that a stepper motor is typically attached directly to and adds directly to the length of the linear actuator shaft as disclosed in Nishi U.S. Pat. No. 3,915,651. Consequently, electrically operated pipettes have not been characterized by portable operation in the past.
A further consideration of portability for pipettes is weight. However, considerable energy is required by known pipettes having an electrically operated linear actuator. For example, in order to hold stepper motors in position, continuous power is typically needed. Heretofore, electrically operated pipettes, such as disclosed in Nishi U.S. Pat. No. 3,915,651, have required such significant amounts of power that power has been supplied by a circuit which is separate from the other components of the instrument. Combination of the circuit and the remainder of the components of known electrically operated pipettes into a self-contained instrument would result in a bulky instrument which would not be portable in any practical sense. Nor have the power demands of known stepper motor circuits heretofore enabled an electrically operated pipette to be battery powered. Further, known stepper motor circuits include loss of torque during high speed movement, a characteristic that can cause loss of step count and consequent imprecise linear actuator movement.
A further difficulty with the known pipette technology is that precise digital movement has not been applied to alleviate inaccuracies inherent in pipetting and/or titrating with a pipette having an electrically operated linear actuator, such as disclosed in Nishi U.S Pat. No. 3,915,651. For example, inaccuracies resulting from surface tension, atmospheric pressure, and expansion and contraction of the air typically found in pipettes have heretofore not been addressed. Furthermore, the configuration of the pipetting displacement assembly provides accuracy only over a limited range, which means that inaccuracy has resulted when the pipette is operated beyond the range.